The Present invention relates to apparatus for magnetron sputtering of a target material onto a substrate; more particularly to such apparatus including an array of unbalanced magnetrons wherein some of the magnetic field lines arising in each magnetron are not closed in the magnetron but either are open or are closed in a different magnetron, and most particularly, to such apparatus wherein magnetron cathodes are arranged in a xe2x80x9cmirrorxe2x80x9d configuration and are driven in pairs by AC power supplies. Apparatus and methods in accordance with the invention are especially useful for magnetron sputter deposition of electrically insulative materials at high rates with significant ion bombardment of the substrate.
Magnetron sputtering is a widely used method for coating relatively thin films of a target material on a substrate surface. Many applications for which magnetron sputtering is particularly well-suited, such as coatings for wear and corrosion resistance and dense dielectric coatings, benefit from a high level of ionization in the vicinity of the substrate being coated. This permits effective use of substrate electrical bias, or creates a level of self-bias on non-conducting substrates, that can improve the properties of the films being deposited through controlled levels of ion bombardment of the coating.
A useful method of increasing plasma density near the substrate(s) is xe2x80x9cunbalancedxe2x80x9d magnetron sputtering, wherein part of the magnetic field, which in balanced magnetrons confines electrons to the vicinity of the target, is allowed to extend away from the target surface toward the substrate(s) on which the material is being deposited. Thus, some of the field lines emanating from the target surface do not close back through the target surface, allowing electrons, which are highly mobile in directions parallel to the magnetic field lines, to be accelerated away from the target, whereby the electrons can desirably ionize the working gas in the vicinity of the substrate. Window and Savvides first described unbalanced magnetrons; see Journal of Vacuum Science and Technology A4, 196 (1986) and Journal of Vacuum Science and Technology A4, 453 (1986).
In prior art unbalanced magnetrons, the xe2x80x9copenxe2x80x9d or unbalanced magnetic field lines can project from either the outside of the target surface or the inside of the target surface; and the targets can be circular, rectangular, or any other desired planar shape. The magnetic polarity is also unimportant, and either the north or the south magnetic pole can be used to create the open or unbalanced field lines in all cases. In some instances, a plurality of unbalanced magnetron cathodes are used in concert to form a plasma trap surrounding a substrate. Such devices are disclosed, for example, in U.S. Pat. Nos. 5,196,105 and 5,556,519. Such a multiple-cathode arrangement is particularly beneficial in large coaters used to deposit hard and/or corrosion-resistant materials, especially on irregular three-dimensional objects. Frequently, the higher plasma densities produced by unbalanced magnetron sputtering are augmented by a negative electrical bias applied to the substrate, which accelerates ions toward the growing film with resulting improvements in density, composition, and microstructure of the deposited material. Therefore, it is highly desirable to provide a relatively high ionization density in the region near the substrate.
It is impotent to note that in all prior art arrays of multiple unbalanced planar magnetron cathodes, there exists a direct magnetic xe2x80x9clinkagexe2x80x9d between the cathodes in the array, and further, that the cathodes are powered with a DC voltage and current, as shown, for example, in FIGS. 1-5 in U.S. Pat. No. 5,196,105. Without such magnetic linkage between cathodes, the plasma density in the region near the substrate is substantially reduced. In contrast, in a xe2x80x9cmirrorxe2x80x9d configuration in which there is no magnetic linkage between opposing cathodes, the ion saturation current is a fraction of what it is in the xe2x80x9copposedxe2x80x9d case with magnetic linkage. See xe2x80x9cEffects of an Unbalanced Magetron in a Unique Dual Cathode, High Rate Readtive Sputtering System,xe2x80x9d by Rhode et al., and published in Thin Solid Films, 193/194 (1990) pp. 117-126. A possible explanation of this phenomenon is that the plasma potential in these DC-driven arrays is negative, which means that in the mirror configuration, electrons migrating along open field lines are lost to the walls of the sputtering chamber before they can produce significant levels of ionization.
Therefore, all prior art arrays of unbalanced planar magnetron cathodes have xe2x80x9copposedxe2x80x9d polar configurations such that the cathodes are magnetically linked. Arrays are known to comprise as many as four or more magnetrons, but in all such prior art magnetron arrays, the cathodes are DC driven and the cathodes are linked by magnetic field lines.
Another sputter coating method in wide use, especially for deposition of insulative materials at high deposition rates, is reactive sputtering using two magnetron cathodes coupled by an alternating current (AC) power supply. See xe2x80x9cdual cathode sputteringxe2x80x9d as disclosed, for example, in U.S. Pat. Nos. 4,041,353; 4,111,782; 4,116,793; 4,116,794; 4,132,612; 4,132,613; 5,082,546; and 5,169,509. A serious problem arises, however, in attempting to combine such AC-driven sputtering with prior art configurations of unbalanced magnetrons to achieve superior coatings. Because of alternating current in the driver, each electrode in each magnetron alternates between being an anode and a cathode with each cycle. While one electrode is attracting ions and being sputtered, the other is attracting electrons. Thus, each xe2x80x9cunbalancedxe2x80x9d magnetic field line extending between cathodes becomes effectively an electrical xe2x80x9cshortxe2x80x9d and so prior art arrays of xe2x80x9copposedxe2x80x9d unbalanced magnetrons cannot use AC sputtering at high plasma densities to enable high rate reactive sputtering of insulative materials. Such conditions would be highly useful in sputter coating to produce, for example, aluminum oxide with excellent properties.
What is needed is a means for utilizing an array of unbalanced magnetrons driven by AC voltage and current to yield a high deposition rate of insulating materials.
Briefly described, an array of unbalance magnetrons is arranged around a centrally-located substrate for sputter coating of material from cathodes in the magnetrons onto the substrate. The electrodes are powered in pairs by an alternating voltage and current source. The unbalanced magnetrons, which may be planar, cylindrical, or conical, are arranged in mirror configuration such that like poles are opposed across the substrate space or are adjacent on the same side of the substrate space. The magnetrons are all identical in magnetic polarity. Because of the mirror configuration, the magnetrons are not capable of magnetic linking with one another, despite being unbalanced. Because the peripheral poles have the greater magnetic cross-sectional area, unclosed lines of flux extend away from the array. Because of the positive plasma potential produced by an AC driver, electrons are prevented from escaping to ground along the unclosed field lines. The positive plasma potential attracts electrons to the vicinity of the substrate, increasing plasma density in the background working gas and thereby improving the quality of coating being deposited on the substrate.